Problem: Christopher is 4 times as old as Umaima and is also 30 years older than Umaima. How old is Umaima?
We can use the given information to write down two equations that describe the ages of Christopher and Umaima. Let Christopher's current age be $c$ and Umaima's current age be $u$ $c = 4u$ $c = u + 30$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $u$ , and both of our equations have $c$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $4u$ $-$ $ (u + 30)$ which combines the information about $u$ from both of our original equations. Solving for $u$ , we get: $3 u = 30$ $u = 10$.